The invention relates to the detection of deterioration of an oxygen sensor disposed in the exhaust gas stream of an internal combustion engine.
The invention arose during development efforts directed toward reducing downtime of large, stationary internal combustion engines continuously operated over long intervals. Such engines generate up to thousands of horsepower, and are used in large scale electrical and motive power generation applications, for example utility company power generation, mining and pumping applications, ocean going vessels, and so on. These engines are characterized by extremely long service intervals, as compared to automotive applications. For example, some of such engines have oil change intervals of 5,000 hours. In contrast, a typical automobile driven 100,000 miles has only been in actual operational service for about 2,000 to 3,000 hours.
During the noted long intervals between service on large engines, it is desirable to allow continuous operation, without downtime. Furthermore, the engine should operate within specified tolerances during the entire length of such interval, without drifting from allowable specifications. One of such specifications is that the proper air/fuel ratio be maintained within an allowable tolerance window. Another specification is that exhaust emissions be maintained below a given limit.
The noted large, long interval engines include an oxygen sensor disposed in the exhaust gas stream, for example, as shown in U.S. Pat. No. 4,638,783. The oxygen sensor detects the relative presence of oxygen in the exhaust gases of the engine and generates an output voltage signal which is fed back to a controller controlling the fuel delivery system to ensure that the proper air/fuel ratio is being supplied to the engine, and also to ensure that the proper exhaust gas constituents are transmitted downstream to a catalytic converter for oxidation and reduction. For rich stoichiometric combustion, it is desired to reduce the oxygen content remaining after combustion to near zero: For example, where methane is the fuel, the stoichiometric combustion process is EQU CH.sub.4 2O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O.
For rich stoichiometric combustion, the air/fuel ratio mixture supplied to the engine is controlled such that any O.sub.2 remaining on the right side of the equation is reduced to near zero. For lean burn combustion, the air/fuel ratio mixture supplied to the engine is controlled such that there is some O.sub.2 remaining after combustion.
The oxygen sensor deteriorates as it ages during operation of the engine. This deterioration alters the voltage output characteristic of the sensor. The altered output characteristic in turn provides a different feedback signal to the fuel control or carburetion system which in turn supplies a different air/fuel ratio to the engine. Because of the altered air/fuel ratio, the engine will no longer be operating within the desired tolerance. The altered air/fuel ratio also changes the constituents in the exhaust gas transmitted to the catalytic converter, which then may not fully oxidize and reduce same.
In order to maintain proper engine operation within acceptable tolerances including intake air/fuel ratio, and in order to ensure that the proper exhaust gas constituents are transmitted downstream to the catalytic converter for reduction, it is necessary to periodically check or test the oxygen sensor for deterioration, and to replace the sensor as needed. In the noted large, long interval engines, it is not desirable to periodically shut down the engine, in order to check the oxygen sensor. The downtime is an economic hardship in most applications. It is thus desirable to provide a method for testing the oxygen sensor during engine operation. Various systems have been proposed for testing the oxygen sensor on-line, but are complex and/or costly.
The present invention provides a particularly simple and effective method and system for testing the oxygen sensor on-line.
The present invention employs a standard oxygen sensor known in the prior art, and uses the known output characteristics thereof in a novel manner, including deterioration characteristics as the sensor ages.